1. Field of the Invention
The present invention relates to a pressure control valve for controlling a pressure on the outlet side of a heat emitter of a vapor compression type refrigerating system. The present invention is preferably used in a vapor compression type refrigerating system using a refrigerant such as carbon dioxide (CO.sub.2) at a super critical region.
2. Description of the Related Art
In recent years, as one of the measures for abolishing the use of fluorocarbon refrigerants in vapor compression type refrigerating systems, for example, a vapor compression type refrigerating system using carbon dioxide (CO.sub.2) (hereinafter referred to simply as a CO.sub.2 system) as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-18602 has been proposed.
This CO.sub.2 system operates in the same way as the vapor compression type refrigerating systems of the related art using fluorocarbons. Namely, as indicated by A-B-C-D-A of FIG. 1 (CO.sub.2 Mollier chart), CO.sub.2 in the gas phase state is compressed by a compressor (A-B), then this high temperature, high pressure super critical state CO.sub.2 is cooled by a heat emitter (gas cooler) (B-C). Next, the pressure is reduced by a decompressor (C-D) to cause the now two-phase gas and liquid state CO.sub.2 to evaporate (D-A), rob the lateral heat of vaporization from the air or other external fluid, and thereby cool the external fluid. Note that, the CO.sub.2 changes to the two-phase gas and liquid state when the pressure falls below a saturated liquid pressure (pressure at intersection of segment CD and a saturated liquid line SL), therefore where the phase slowly changes from the state of C to the state of D, the CO.sub.2 changes from the super critical state to the two phase gas and liquid state through a liquid phase state.
Note that the "super critical state" means a state where the CO.sub.2 molecules behave as if in the gas phase state even though the density is substantially equivalent to the liquid density.
However, the critical temperature of CO.sub.2 is about 31.degree. C. which is lower than the critical temperature of the fluorocarbons of the related art (for example, 112.degree. C. in the case of R12), therefore the CO.sub.2 temperature at the heat emitter side ends up becoming higher than the critical point temperature of CO.sub.2 in the summer etc. That is, in this state, CO.sub.2 is not condensed even at the outlet side of the heat emitter (the line segment BC does not intersect the saturated liquid line SL).
Further, the state at the outlet side of the heat emitter (C point) is determined by the outlet pressure of the compressor and the CO.sub.2 temperature at the outlet side of the heat emitter, while the CO.sub.2 temperature at the outlet side of the heat emitter is determined by the heat emitting capability of the heat emitter and the external temperature. However, the external temperature cannot be controlled, therefore the CO.sub.2 temperature at the outlet side of the heat emitter cannot be controlled in practice.
Accordingly, the control of the state at the outlet side of the heat emitter (C point) becomes possible by controlling the outlet pressure of the compressor (or the pressure at the outlet side of the heat emitter). That is, where the external temperature is high as in the summer, in order to secure a sufficient cooling capability (enthalpy difference), as indicated by E-F-G-H-E of FIG. 1, it is necessary to make the pressure at the outlet side of the heat emitter high.
However, since the outlet pressure of the compressor must be made high in order to make the pressure at the outlet side of the heat emitter high as mentioned above, the compression work of the compressor (change of enthalpy .DELTA.L of the compression step) is increased. Accordingly, where the increase of the change of enthalpy .DELTA.L of the compression step (A-B) is larger than the increase of the change of enthalpy .DELTA.i of the evaporation step (D-A), the coefficient of performance of the CO.sub.2 system (COP=.DELTA.i/.DELTA.L) is degraded.
Therefore, if the CO.sub.2 temperature at for example the outlet side of the heat emitter is set to 40.degree. C. and a trial calculation is made of the relationship between the CO.sub.2 pressure at the outlet side of the heat emitter and the coefficient of performance using FIG. 1, as indicated by a solid line in FIG. 2, the coefficient of performance becomes maximum at a pressure P.sub.1 (about 10 MPa). Similarly, where the CO.sub.2 temperature at the outlet side of the heat emitter is set to 35.degree. C., as indicated by a broken line of FIG. 2, the coefficient of performance becomes maximum at a pressure P.sub.2 (about 9.0 MPa).
As described above, when the CO.sub.2 temperature at the outlet side of the heat emitter and the pressure at which the coefficient of performance becomes maximum are calculated and the result of this is plotted in FIG. 1, the relationship becomes as indicated by the bold solid line .eta..sub.max of FIG. 1 (hereinafter referred to as the optimum control line).
Accordingly, in order to operate the CO.sub.2 system at a high efficiency, it is necessary to provide a pressure control valve for controlling the pressure at the outlet side of the heat emitter and the CO.sub.2 temperature at the outlet side of the heat emitter as indicated by the optimum control line .eta..sub.max.
Note that the Mollier chart of FIG. 1 is taken from the Fundamentals Handbook published by the American Society of Heating, Refrigerating and Air-conditioning Engineers.